The other day we had our water heater go out and the issue was diagnosed to a faulty thermocouple. When the repair man removed the faulty thermocouple he noticed the following disk-like component in series about mid-way down the copper tubing.
This device had one of its two terminals broken off of the bottom (not shown) and was most certainly the cause for the failure. The repair man's replacement thermocouple had no such device inline and when asked what this was he replied it was a transistor. I very much doubt this device is a transistor as there are only two, not three, terminals but I didn't want to argue.
So, what exactly is this component and what is it's function? As stated, the replacement thermocouple does not have this but it appears to be functioning correctly.
Best Answer
If I was a betting man, I'd suggest that it's a low-melting-point alloy such as Wood's Alloy designed to melt and open the circuit from the thermopile (thus closing the gas valve) in case of a house fire or fire under the water heater, but I have no evidence to support that premise!
Possibly 180°C is the melting point.
Edit: Okay, evidence gathered. See Amazon review here,
and this Amazon photo (which is the item referenced in the review).
You might want to discuss this with your repair service company- it appears they may have compromised the overall safety by replacing the original safety device with an inferior generic replacement. It also sounds like the guy they sent out was a dolt- "transistor"!